Current emission control regulations necessitate the use of catalysts in the exhaust systems of automotive vehicles in order to convert carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) produced during engine operation into harmless exhaust gasses. Vehicles equipped with diesel or lean gasoline engines offer the benefits of increased fuel economy. Such vehicles have to be equipped with lean exhaust aftertreatment devices, such as, for example, an Active Lean NOx Catalysts (ALNC) or Selective Catalytic Reduction (SCR) catalysts, which continuously reduce NOx emissions, even in an oxygen rich environment, through active injection of reductant, such as fuel (HC) or urea, into the exhaust gas entering these devices. Further, it is important to precisely control the amounts of reductant and to optimize its delivery methods in order to achieve maximum NOx conversion efficiency.
The inventors herein have recognized NOx conversion efficiency of a lean exhaust gas aftertreatment device can be improved by adjusting the amounts of reductant injected to account for variation in feedgas NOx production due to engine transient behaviors, such as changes in the pedal position, or the EGR valve position. For example, NOx production generally increases during pedal tip-ins, and therefore, the amount of reductant supplied to the exhaust gas aftertreatment device may be increased to compensate for additional NOx produced. Additionally, the inventors have recognized that mixing the adjusted amount of reductant with air and vaporizing the mixture prior to introducing it into the device can further improve NOx conversion efficiency of the device.
In accordance with the present invention, a system and a method for controlling an amount of reductant to be delivered to an exhaust gas aftertreatment device are presented. The method includes calculating a desired amount of reductant based on a measure of engine transient behavior; injecting said calculated desired amount of reductant into a heated reductant delivery system; injecting air into said reductant delivery system thereby creating a vaporized mixture of said injected reductant and said air; and directing said vaporized mixture into the exhaust gas aftertreatment device. In one aspect of the present invention, the device is an ALNC and the reductant is hydrocarbon. In another aspect of the present invention, the measure of engine transient behavior is an instantaneous change in pedal position. In yet another aspect of the present invention, the device is an SCR catalyst and the reductant is urea.
The present invention provides a number of other advantages. In particular, NOx conversion efficiency of the exhaust gas aftertreatment device is improved by adjusting the injected reductant amounts to compensate for transient increases or decreases in the engine feedgas NOx amounts Another advantage of the present invention is improved fuel economy due to optimized reductant usage. Yet another advantage of the present invention is that NOx conversion efficiency is further improved by using an air assist heated reductant delivery system to deliver reductant to the exhaust gas aftertreatment device.
The above advantages and other advantages, objects and features of the present invention will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings.